1. Field of Invention
This invention relates to a motor control system, and more particularly, to a DC motor speed control system having a simple current limiting circuit for limiting excess current to a DC motor from a DC power source.
2. Description of the Prior Art
In a conventional speed control system for a DC motor, wherein drive current to the DC motor is controlled by chopping signals, a current limiting circuit is provided in order to prevent the motor and DC power source from buring when excess current (or sometimes called overcurrent) flows continuously.
In such a prior current limiting circuit; however, disadvantageously, if a large load is momentarily applied to a DC motor during starting or driving, the current limiting circuit is caused to operate, and consequently, current above a predetermined value will not flow in the motor and torque of the motor cannot be increased during that time.
Moreover, in the conventional current limiting circuit, overcurrent to a motor is detected during conduction of a power control element, such as a power transistor. Since the overcurrent may vary near a predetermined limit value, a high frequency switching signal, in synchronization with the frequency of the current variation, is applied to the power transistor, by the current limiting circuit. However, the power transistor cannot respond accurately to the high frequency signal. Thus, overcurrent may flow in the motor continuously. As a result, current limitation is not reliably effected by the prior current limiting circuit and the motor and power transistor may be burned, or in certain circumstances, the power source transformer may also be burned.
Thus, the prior art DC motor control systems leave something to be desired, and improvement is greatly desired.